Single crystal ferrimagnetic films

ABSTRACT

A process for preparing single crystal ferrimagnetic films having square hysteresis loops, a regulated coercivity, and a compensation temperature above 250* K. A film of gadolinium iron garnet wherein a carefully selected amount of terbium or dysprosium replaces part of the gadolinium is deposited on a tytrium or rare earth aluminum garnet substrate having (111) or (100) orientation and heated until crystallization occurs.

United States Patent Richard L. Comstock San Jose;

Daniel A. Nepela, Saratoga, both of Calif. 862,080

Sept. 29, 1969 Sept. 21, 1971 International Business MachinesCorporation Armonk, N.Y.

Inventors Appl. No. Filed Patented Assignee SINGLE CRYSTAL FERRIMAGNETICFILMS 5 Claims, No Drawings u.s.c| 117/237,

117/235 Int. Cl n01: 10/02 FieldolSearch 117/235,

Primary Examiner-William D. Martin Assistant Examiner- Bernard D.Pianalto AnorneysHanifin and Jancin and Joseph G. Walsh ABSTRACT: Aprocess for preparing single crystal ferrimagnetic films having squarehysteresis loops, a regulated coercivity, and a compensation temperatureabove 250 K. A film of gadolinium iron garnet wherein a carefullyselected amount of terbium or dysprosium replaces part of the gadoliniumis deposited on a tytrium or rare earth aluminum garnet substrate having[111] or [100] orientation and heated until crystallization occurs,

SINGLE CRYSTAL FERRIMAGNETIC FILMS FIELD OF THE INVENTION The presentinvention relates to a process for making single crystal films and moreparticularly to a process for making thin single crystal garnet filmshaving ferrimagnetic properties and the advantage of simultaneouslycombining square hysteresis loops, controllable coercivity, andcompensation temperatures near room temperature. The preparation of suchfilms is very important, since the films are useful as modulators forlight beams, as couplers or modulators in microwave systems, and as theactive memory elements in beam addressable memory systems such asmagnetothermal recording with magneto-optical readout.

PRIOR ART Pending application Ser. No. 766,698, filed Oct. 1 l, 1968, byEric Kay and Erich Sawatzky, and assigned to the assignee of the presentapplication, discloses a process for forming single crystal garnetfilms. According to that application, gadolinium iron garnet isdeposited on a substrate of yttrium aluminum garnet in the [1 1 l]orientation.

SUMMARY OF THE INVENTION The present application differs from the priorone in that a portion of the gadolinium must be replaced by eitherterbium or dysprosium. The substrate, besides being yttrium aluminumgarnet, may also be rare earth aluminum garnet. Furthermore, thesubstrate orientation may also be [100] as well as [l 1 1]. Thesubstrate orientations are indicated according to standardcrystallographic practice. See, for example, the textbook, STRUCTURE OFMETALS, by Barrett, McGraw Hill Book Company, Inc., Second Edition,1952, pages 8-9. According to the present invention, the garnetsubstrate has either [100] or [1 1 1] orientation, and the formula R-,A1,,O wherein R is yttrium or a rare earth. The film has thecomposition Gd ,M I Fe O wherein M is terbium or dysprosium and x iswithin the range of about from 0.1 to 0.6 when M is terbium and theorientation is [111], within the range of about from 1 to 2.7 when M isterbium and the orientation is [100], within the range of about 0.1 to1.7 when M is dysprosium and the orientation is [11 1], and within therange of about 0.27 to 1.7 when M is dysprosium and the orientation is[100].

In carrying out the process of the present invention, the deposition ofthe film on the substrate can be accomplished by any of several methods.Useful methods include spinning from an aqueous system of the propercomposition and subsequent conversion to a crystalline state, and alsoinclude reactively sputtering the appropriate composition wherein thecomposition to be deposited constitutes the cathode of the sputteringsystem. The technique of sputtering with two cathodes can also be used,as described in copending application Ser. No. 818,588, filed Apr. 23,1969, by Eric Kay and Erich Sawatzky.

The techniques employed for the steps of the present invention are thesame as those which have been previously described in the literature.The chemical method of spinning is preferred because of the ease inchanging composition and because the resulting films have particularlygood optical quality. According to this technique, a concentrated stocksolution containing gadolinium and iron ions and also those of eitherterbium or dysprosium (preferably all as nitrates) is first dilutedwith, for example, ethyl alcohol. A few drops of the diluted solutionare applied to the prepared substrate held horizontally on a photoresistspinner. The solution is dis tributed into a uniform film by spinning.After 15-20 seconds of spinning, the film is sufficiently dry to remainquiescent on the substrate surface. Further drying on a hot plate at3005O 0 C. decomposes the nitrates to oxides and converts the film to aglassy state which makes the film insoluble in a spinning solution. Thespin dry process may then be repeated to achieve the desired finalthickness. Single coat thicknesses are about 1,000 A.The dry product isan amorphous solid which has the desired composition according to theformula Gd ,,M

,Fe O given above, and wherein the amount of gadolinium, iron, terbium,or dysprosium is in accordance with the relative amounts present in theoriginal solutions. The crystalline garnet is obtained by heating in thepresence of oxygen, for example, in the ambient atmosphere. Atemperature of about 650 C. to 800 C. is suitable for crystallization.

We have discovered that films having useful properties may be obtainedby careful selection of the proper amount of terbium or dysprosiumsubstituted for some of the gadolinium. It is essential that the properamount of substituent be used. The proper amount is different forterbium from that for dysprosium, and it also depends upon theorientation of the substrate. When terbium is used and the substrate hasthe 1 1 l] orientation, the value ofx in the formula Gd M Fe O isbetween 0.1 and 0.6. When the upper limit is exceeded, the hysteresisloop is no longer square. On the other hand, when the amount of terbiumis below this limit, coercivity may be too high. When the substrateorientation is the value of x is between 1 and 2.7. When the content isbelow this limit, the hysteresis loop is no longer square. On the otherhand, when it is above the limit, the compensation temperature is toolow. When the substituent is dysprosium the value of x should be between0.1 and 1.7 when the orientation of the substrate is [111]. When thedysprosium content is too low, it does not exert sufficient influence onthe coercivity. When the dys prosium content is too high, thecompensation temperature becomes too low. When the orientation is 100]the value ofx should be between 0.27 and 1.7. When the dysprosium levelis below this limit, the hysteresis loop is no longer square and whenthe dysprosium content is above this limit, the compensation temperaturedrops too low.

The present invention provides a method of obtaining thin ferrimagneticfilms having square hysteresis loops, compensation temperatures in themost desirable range near ambient temperatures, and also coercivitieswhich may be regulated within a wide range of useful values. These threephysical properties are simultaneously determined according to thepresent invention by the proper selection of the amount of terbium ordysprosium.

PREFERRED EMBODIMENTS The general nature of the invention having beenset forth, the following examples are now presented as to the specificpreparation of preferred embodiments of the invention. The specificdetails presented are for purposes of illustration and not limitation.

Example 1 Gd -,Tb Fe O, on [1 1 l oriented yttrium aluminum garnetyields a coercivity of 250 0c at compensation temperature plus 30 C.,and an Mr/Ms=l .0.

Example 11 Gd Tb Fe O, on [11 1] oriented yttrium aluminum garnet yieldsa coercivity of oe at compensation temperature plus 30 C. and anMr/Ms=1.0.

Example 111 Gd Tb Fe O on [100] oriented yttrium aluminum garnet yieldsa coercivity of 520 oe at compensation temperature plus 30 C. and anMr/Ms Z 0.95.

Example IV Gd Dy,Fe O, on [111] oriented yttrium aluminum garnet yieldsa coercivity of 1,140 oe at compensation temperature plus 30 C. and anMr/Ms=l .0.

Example V Gd Dy Je O on [100] oriented yttrium aluminum garnet wouldyield a coercivity of 520 at compensation temperature plus 30 C. and anMr/Ms Z 0.95.

Example VI In a manner like that shown in the above examples, in placeof yttrium aluminum garnet substrate, rare earth aluminum garnetsubstrate may be employed and similar results obtained.

All of the above films were prepared at a thickness of l.0p.i0.2.

While the invention has been shown and described with reference topreferred embodiments thereof, it will be ap preciated by those skilledin the art that variations in form may be made therein without departingfrom the spirit and scope of the invention.

What is claimed is:

l. A process for preparing a single crystal ferrimagnetic film having asquare hysteresis loop, a regulated coercivity, and a compensationtemperature above 250 K., said process comprising:

a. depositing on a garnet substrate having [111] or [100] orientation,and having the formula R Al O wherein R is yttrium or a rare earth, afilm having the composition Gd -,,,M,Fe .,0,, wherein M is terbium ordysprosium and x is within the range from 0.1 to 0.6 when M is terbiumand the orientation is [l l l within the range of l to 2.7

when M is terbium and the orientation is [I00], within the range 0.1 to1.7 when M is dysprosium and the orientation is [l l 1], and within therange 0.27 to 1.7 when M is dysprosium and the orientation is and b.heating in an oxygen-containing atmosphere until crystallization occurs.

2. A process as claimed in claim 1 wherein the substrate is yttriumaluminum garnet having [111] orientation and the film has the formula Gd,,Tb,Fe O wherein x is within the range from 0.1 to 0.6.

3. A process as claimed in claim 1 wherein the substrate is yttriumaluminum garnet having [100] orientation and the film has the formula GdTb,Fe O wherein x is within the range from 1 to 2.7.

4. A process as claimed in claim 1 wherein the substrate is yttriumaluminum garnet having [111] orientation and the film has the formula GdDy,Fe O,, wherein x is within the range from 0.1 to 1.7.

5. A process as claimed in claim 1 wherein the substrate is yttriumaluminum garnet having [100] orientation and the film has the formula Gd,,Dy ,Fe -,O, wherein x is within the range from 0.27 to 1.7.

2. A process as claimed in claim 1 wherein the substrate is yttriumaluminum garnet having (111) orientation and the film has the formulaGd3 xTbxFe5O12 wherein x is within the range from 0.1 to 0.6.
 3. Aprocess as claimed in claim 1 wherein the substrate is yttrium aluminumgarnet having (100) orientation and the film has the formula Gd3xTbxFe5O12 wherein x is within the range from 1 to 2.7.
 4. A process asclaimed in claim 1 wherein the substrate is yttrium aluminum garnethaving (111) orientation and the film has the formula Gd3 xDyxFe5O12wherein x is within the range from 0.1 to 1.7.
 5. A process as claimedin claim 1 wherein the substrate is yttrium aluminum garnet having (100)orientation and the film has the formula Gd3 xDyxFe5O12 wherein X iswithin the range from 0.27 to 1.7.